Oscillators are often characterized by the frequency of their output signal:

A low-frequency oscillator (LFO) is an electronic oscillator that generates a frequency below approximately 20 Hz. This term is typically used in the field of audio synthesizers, to distinguish it from an audio frequency oscillator.

An audio oscillator produces frequencies in the audio range, about 16 Hz to 20 kHz.[2]

An RF oscillator produces signals in the radio frequency (RF) range of about 100 kHz to 100 GHz.[2]

Oscillators designed to produce a high-power AC output from a DC supply are usually called inverters.

There are two main types of electronic oscillator – the linear or harmonic oscillator and the nonlinear or relaxation oscillator.[2][3]

1 MHz electronic oscillator circuit which uses the resonant properties of an internal quartz crystal to control the frequency. Provides the
clock signal for digital devices such as computers.

Harmonic oscillator

Block diagram of a feedback linear oscillator; an amplifier
A with its output
vo fed back into its input
vf through a
filter,
β(jω).

Two common LC oscillator circuits, the Hartley and Colpitts oscillators

In a crystal oscillator circuit the filter is a piezoelectric crystal (commonly a quartz crystal).[2][3] The crystal mechanically vibrates as a resonator, and its frequency of vibration determines the oscillation frequency. Crystals have very high Q-factor and also better temperature stability than tuned circuits, so crystal oscillators have much better frequency stability than LC or RC oscillators. Crystal oscillators are the most common type of linear oscillator, used to stabilize the frequency of most radio transmitters, and to generate the clock signal in computers and quartz clocks. Crystal oscillators often use the same circuits as LC oscillators, with the crystal replacing the tuned circuit;[2] the Pierce oscillator circuit is also commonly used. Quartz crystals are generally limited to frequencies of 30 MHz or below.[2] Other types of resonator, dielectric resonators and surface acoustic wave (SAW) devices, are used to control higher frequency oscillators, up into the microwave range. For example, SAW oscillators are used to generate the radio signal in cell phones.

Negative-resistance oscillator

(left) Typical block diagram of a negative resistance oscillator. In some types the negative resistance device is connected in parallel with the resonant circuit.
(right) A negative-resistance microwave oscillator consisting of a
Gunn diode in a
cavity resonator. The negative resistance of the diode excites microwave oscillations in the cavity, which radiate out the aperture into a
waveguide.

In addition to the feedback oscillators described above, which use two-port amplifying active elements such as transistors and operational amplifiers, linear oscillators can also be built using one-port (two terminal) devices with negative resistance,[2][3] such as magnetron tubes, tunnel diodes, IMPATT diodes and Gunn diodes. Negative-resistance oscillators are usually used at high frequencies in the microwave range and above, since at these frequencies feedback oscillators perform poorly due to excessive phase shift in the feedback path.

In negative-resistance oscillators, a resonant circuit, such as an LC circuit, crystal, or cavity resonator, is connected across a device with negative differential resistance, and a DC bias voltage is applied to supply energy. A resonant circuit by itself is "almost" an oscillator; it can store energy in the form of electronic oscillations if excited, but because it has electrical resistance and other losses the oscillations are damped and decay to zero. The negative resistance of the active device cancels the (positive) internal loss resistance in the resonator, in effect creating a resonator with no damping, which generates spontaneous continuous oscillations at its resonant frequency.

The negative-resistance oscillator model is not limited to one-port devices like diodes; feedback oscillator circuits with two-port amplifying devices such as transistors and tubes also have negative resistance.[4][5][6] At high frequencies, transistors and FETs do not need a feedback loop, but with certain loads applied to one port can become unstable at the other port and show negative resistance due to internal feedback, causing them to oscillate.[4][5][7] So high-frequency oscillators in general are designed using negative-resistance techniques.[4][5][6]

Some of the many harmonic oscillator circuits are listed below:

Active devices used in oscillators and approximate maximum frequencies
[5]

Ring oscillators are built of a ring of active delay stages. Generally the ring has an odd number of inverting stages, so that there is no single stable state for the internal ring voltages. Instead, a single transition propagates endlessly around the ring.

Some of the more common relaxation oscillator circuits are listed below:

Voltage-controlled oscillator (VCO)

An oscillator can be designed so that the oscillation frequency can be varied over some range by an input voltage or current. These voltage controlled oscillators are widely used in phase-locked loops, in which the oscillator's frequency can be locked to the frequency of another oscillator. These are ubiquitous in modern communications circuits, used in filters, modulators, demodulators, and forming the basis of frequency synthesizer circuits which are used to tune radios and televisions.

Radio frequency VCOs are usually made by adding a varactor diode to the tuned circuit or resonator in an oscillator circuit. Changing the DC voltage across the varactor changes its capacitance, which changes the resonant frequency of the tuned circuit. Voltage controlled relaxation oscillators can be constructed by charging and discharging the energy storage capacitor with a voltage controlled current source. Increasing the input voltage increases the rate of charging the capacitor, decreasing the time between switching events.

History

The first practical oscillators were based on electric arcs, which were used for lighting in the 19th century. The current through an arc light is unstable due to its negative resistance, and often breaks into spontaneous oscillations, causing the arc to make hissing, humming or howling sounds[8] which had been noticed by Humphry Davy in 1821, Benjamin Silliman in 1822,[9]Auguste Arthur de la Rive in 1846,[10] and David Edward Hughes in 1878.[11]Ernst Lecher in 1888 showed that the current through an electric arc could be oscillatory.[12][13][14] An oscillator was built by Elihu Thomson in 1892[15][16] by placing an LC tuned circuit in parallel with an electric arc and included a magnetic blowout. Independently, in the same year, George Francis FitzGerald realized that if the damping resistance in a resonant circuit could be made zero or negative, the circuit would produce oscillations, and, unsuccessfully, tried to build a negative resistance oscillator with a dynamo, what would now be called a parametric oscillator.[17][8] The arc oscillator was rediscovered and popularized by William Duddell in 1900.[18][19] Duddell, a student at London Technical College, was investigating the hissing arc effect. He attached an LC circuit (tuned circuit) to the electrodes of an arc lamp, and the negative resistance of the arc excited oscillation in the tuned circuit.[8] Some of the energy was radiated as sound waves by the arc, producing a musical tone. Duddell demonstrated his oscillator before the London Institute of Electrical Engineers by sequentially connecting different tuned circuits across the arc to play the national anthem "God Save the Queen".[8] Duddell's "singing arc" did not generate frequencies above the audio range. In 1902 Danish physicists Valdemar Poulsen and P. O. Pederson were able to increase the frequency produced into the radio range by operating the arc in a hydrogen atmosphere with a magnetic field, inventing the Poulsen arcradio transmitter, the first continuous wave radio transmitter, which was used through the 1920s.[20][21][22]

A 120 MHz oscillator from 1938 using a parallel rod
transmission line resonator (
Lecher line). Transmission lines are widely used for UHF oscillators.

The vacuum-tube feedback oscillator was invented around 1912, when it was discovered that feedback ("regeneration") in the recently invented audionvacuum tube could produce oscillations. At least six researchers independently made this discovery, although not all of them can be said to have a role in the invention of the oscillator.[23][24] In the summer of 1912, Edwin Armstrong observed oscillations in audion radio receiver circuits[25] and went on to use positive feedback in his invention of the regenerative receiver.[26][27] Austrian Alexander Meissner independently discovered positive feedback and invented oscillators in March 1913.[25][28]Irving Langmuir at General Electric observed feedback in 1913.[28] Fritz Lowenstein may have preceded the others with a crude oscillator in late 1911.[29] In Britain, H. J. Round patented amplifying and oscillating circuits in 1913.[25] In August 1912, Lee De Forest, the inventor of the audion, had also observed oscillations in his amplifiers, but he didn't understand its significance and tried to eliminate it[30][31] until he read Armstrong's patents in 1914,[32] which he promptly challenged.[33] Armstrong and De Forest fought a protracted legal battle over the rights to the "regenerative" oscillator circuit[33][34] which has been called "the most complicated patent litigation in the history of radio".[35] De Forest ultimately won before the Supreme Court in 1934 on technical grounds, but most sources regard Armstrong's claim as the stronger one.[31][33]

The first and most widely used relaxation oscillator circuit, the astable multivibrator, was invented in 1917 by French engineers Henri Abraham and Eugene Bloch.[36][37][38] They called their cross-coupled, dual-vacuum-tube circuit a multivibrateur, because the square-wave signal it produced was rich in harmonics,[37][38] compared to the sinusoidal signal of other vacuum-tube oscillators.

Vacuum-tube feedback oscillators became the basis of radio transmission by 1920. However, the triode vacuum tube oscillator performed poorly above 300 MHz because of interelectrode capacitance.[citation needed] To reach higher frequencies, new "transit time" (velocity modulation) vacuum tubes were developed, in which electrons traveled in "bunches" through the tube. The first of these was the Barkhausen–Kurz oscillator (1920), the first tube to produce power in the UHF range. The most important and widely used were the klystron (R. and S. Varian, 1937) and the cavity magnetron (J. Randall and H. Boot, 1940).

Mathematical conditions for feedback oscillations, now called the Barkhausen criterion, were derived by Heinrich Georg Barkhausen in 1921. The first analysis of a nonlinear electronic oscillator model, the Van der Pol oscillator, was done by Balthasar van der Pol in 1927.[39] He showed that the stability of the oscillations (limit cycles) in actual oscillators was due to the nonlinearity of the amplifying device. He originated the term "relaxation oscillation" and was first to distinguish between linear and relaxation oscillators. Further advances in mathematical analysis of oscillation were made by Hendrik Wade Bode and Harry Nyquist[40] in the 1930s. In 1969 K. Kurokawa derived necessary and sufficient conditions for oscillation in negative-resistance circuits,[41] which form the basis of modern microwave oscillator design.[7]

^US 500630, Thomson, Elihu, "Method of and Means for Producing Alternating Currents", published 18 July 1892, issued 4 July 1893

^G. Fitzgerald, On the Driving of Electromagnetic Vibrations by Electromagnetic and Electrostatic Engines, read at the January 22, 1892 meeting of the Physical Society of London, in Larmor, Joseph, ed. (1902). The Scientific Writings of the late George Francis Fitzgerald. London: Longmans, Green and Co. pp. 277–281.

^GB 190021629, Duddell, William du Bois, "Improvements in and connected with Means for the Conversion of Electrical Energy, Derived from a Source of Direct Current, into Varying or Alternating Currents", published 29 Nov 1900, issued 23 Nov 1901

Morse, A. H. (1925), Radio: Beam and Broadcast: Its story and patents, London: Ernest Benn. History of radio in 1925. Oscillator claims 1912; De Forest and Armstrong court case cf p. 45. Telephone hummer/oscillator by A. S. Hibbard in 1890 (carbon microphone has power gain); Larsen "used the same principle in the production of alternating current from a direct current source"; accidental development of vacuum tube oscillator; all at p. 86. Von Arco and Meissner first to recognize application to transmitter; Round for first transmitter; nobody patented triode transmitter at p. 87.